This invention relates generally to the field of chemical munitions and more particularly to devices of this character wherein a container of pressurized liquid fuel is ruptured on impact. The liquid fuel thereupon vaporizes, disperses, and mixes with air in the form of a cloud which is then detonated by small high explosive charges. "Detonation" is defined herein as an exothermic reaction which propagates at supersonic velocity. "Explosion" is defined as a rapid exothermic reaction at subsonic velocities. In the discussion that follows it will be understood that "detonation" of a fuel-air cloud is always the desired goal, rather than "explosion" of such cloud which would be accompanied by overpressures of lesser magnitude. Detonation occurs only over an area in which the volume percent of fuel in air in the cloud is within certain fairly well defined upper and lower limits generally referred to as the "detonability limits." Ordinarily a time delay of a few seconds is introduced between impact of the device and detonation of the cloud to allow for fuel dispersal time and for the cloud to grow in size over some desired area. Any factor which affects fuel dispersal or cloud growth may inhibit the development of the desired fuel-air ratio over this area. In confined spaces, for example, such as densely forested areas wherein cloud growth is impeded, a relatively fuel-rich mixture may still persist at the time established for cloud detonation. If the volume percent of fuel in such a cloud exceeds the upper detonability limit of the fuel, then no detonation will occur and the device is not completely successful even though the possibility of burning or explosion may still exist.
This illustrates a disadvantage of a prior art device of the character described herein which has employed propane as the detonable fuel. The success of such a device is found to be limited in part because the detonability limits of propane-air mixtures are between 3 and 8% by volume. A fuel-rich cloud might easily exhibit a fuel volume percent substantially greater than 8%. Obviously, therefore, it would be an advantage to substitute for propane a fuel with a higher upper detonability limit.
Unfortunately, fuels which possess a high upper detonability limit tend to be thermodynamically unstable and their equilibrium vapor phases will usually be flammable even in the absence of air at room temperature (approximately 27.degree. C.) and above. In filling the tank of devices of the character to be described, fuel is normally introduced in sufficient quantity and at a pressure such that its liquid phase occupies the substantial proportion, say, at least 90% of the available space in the tank, and the vapor phase in equilibrium occupies the remainder. It is important that this vapor phase shall be nonflammable at temperatures normally to be encountered in handling and storage.
This problem requires the addition of diluents to the primary fuel composition in order to render the vapor phase nonflammable. The function of a diluent is to act as an energy sink in the event of decomposition of the primary fuel composition. If present in sufficient quantities in the vapor phase the diluent will render the fuel nonflammable over a desired range of temperatures and pressures. For example, one such fuel considered for use in a device as described herein is an intermixture of hydrocarbons consisting of methylacetylene, propadiene, or mixtures thereof (hereinafter designated, in any proportions, as C.sub.3 H.sub.4) together with a substantial quantity (up to about 38%) of a mixture of other hydrocarbons, primarily propylene and propane. This fuel has a higher fuel-rich (upper) detonability limit than propane due primarily, it is felt, to the presence of the unsaturated hydrocarbon C.sub.3 H.sub.4. Although the vapor phase of C.sub.3 H.sub.4 is known to be flammable at room temperature and above, with the presence of sufficient quantities of propane or propylene such vapor phase can be rendered nonflammable at such temperature.
The presence of a propane or propylene diluent in such large quantities has the important drawback, however, that it substantially reduces the upper detonability limit otherwise obtainable with C.sub.3 H.sub.4 fuel-air mixtures alone. Thus if a fuel composition can be devised employing C.sub.3 H.sub.4 as the primary constituent in the liquid phase while substantially decreasing the mole percent of diluent required in the liquid phase to stabilize the gas phase, a significant improvement can be expected in the upper detonability limits of the fuel-air mixtures obtainable.
In devices of this type the energy required for initiating a detonation of the fuel-air cloud is also a matter of concern. It is obviously advantageous to keep such energy to a reasonably low level. Further improvement in the device to be described can be achieved with a reduction of the present energy requirements for initiating a detonation of fuel-air clouds resulting from liquid propane fuel or C.sub.3 H.sub.4 plus substantial quantities of propane or propylene diluents. It is recognized that if the liquid fuel can be altered to consist of C.sub.3 H.sub.4 in a more pure form, the energy requirement for initiating fuel-air cloud detonation can be substantially lowered.